Truncation Mutation in HRG4 (UNC119) Leads to Mitochondrial ANT-1–Mediated Photoreceptor Synaptic and Retinal Degeneration by Apoptosis

Abstract

PURPOSE: To characterize the time course of apoptosis and degeneration in a transgenic mouse model of retinal degeneration based on truncated mutant HRG4; to investigate the nature of binding of the mutant HRG4 to its target, ADP-ribosylation factor-like (ARL)2; to study its effects on the downstream molecules Binder-of-ARL2 (BART) and adenine nucleotide transporter (ANT)-1 and on the induction of apoptosis. METHODS: Saturation binding, microscopic morphometric, Western blot, immunofluorescence, and TUNEL analyses were used. RESULTS: Increased apoptosis did not occur until 20 months in the transgenic retina, consistent with the delayed-onset degeneration in this model. The truncated HRG4 protein exhibited approximately threefold greater affinity for ARL2 than the wild-type HRG4, likely resulting in nonfunctional sequestration of ARL2. A significant decrease in ARL2 was present by 20 months, accompanied by a 50% decrease in ANT-1 in the photoreceptor synaptic mitochondria, with evidence of mitochondrial dysfunction. Preapoptotic degeneration in the photoreceptor synapse was demonstrated with cytochrome c release and caspase 3 activation within the synapse-without evidence of TUNEL-positive apoptosis in the photoreceptor cell body-indicating an initial event in the synapse leading to apoptosis. Caspase 3 was activated in the accompanying secondary neuron, consistent with transsynaptic degeneration. CONCLUSIONS: The results support a novel mechanism of retinal degeneration in which preapoptotic degeneration starts in the photoreceptor synapse because of a deficiency in ANT-1 and spreads to the secondary neuron transsynaptically, followed by apoptosis and degeneration in the cell body of the photoreceptor.